This invention relates to a torch assembly for use in a vapor axial deposition (VAD) process in forming optical fiber glass preforms.
In the fabrication of optical fibers, it is customary to create a preform, which is a long glass rod having a central core and which is, in effect, a magnified or enlarged version of the optical fiber to be drawn therefrom. The preform consists of an inner core and an external cladding having an index of refraction profile that reproduces the index profile of the drawn fiber.
There are three major processes for making preforms, the modified chemical vapor deposition (MCVD) process, the outside vapor deposition (OVD) process, and the vapor axial deposition (VAD) process. All of these processes utilize vapor deposition, in which a xe2x80x9csootxe2x80x9d is deposited on the surface of a starting object such as the interior surface of a starter tube or a starting or target rod to form a glassy layer on the surface.
It is with the VAD process that the present invention is primarily concerned, although it is to be understood that the principles and features of the invention are adaptable to other processes as well.
In the VAD process, silica vapors flowing through a heating torch deposit the desired silica particle layers on the rod. As the sooty particles build up to a desired diameter, the target rod is moved upward to make room for further growth, while the torch or torches are fixed in position. When the build-up or deposition is complete, the rod is removed and the resulting preform is sintered or consolidated into a solid preform.
Although the VAD process is widely used, in a production milieu certain problems arise which prevent complete satisfaction with the method, and more particularly, the apparatus. Thus, the glass torches, which must be precisely set for satisfactory and reproducible results, tend, over an extended period of use to become leaky, unstable, and not very consistent in producing repeatable results. For example, the various gases and vapors fed to the torch are generally transmitted through plastic tubing to Teflon fittings which, in turn, connect to the glass torch nipples. These Teflon fittings tend to loosen up over extended periods of use, which can, and often does, give rise to leakage, thereby destroying the calibrated delivery of the gases and vapors to the torch. The tendency to loosen is due, at least in part, to temperature fluctuations and also to lack of strain relief in the plastic delivery tubing. Lack of strain relief is a very important problem, since even small mechanical stress on the tubing can loosen the Teflon to glass interface and cause leaks. For example, during routine machine maintenance, the mechanic can accidentally push the tubing and thereby loosen the interface. A further problem, which can actually be more serious in that it can shut down the production line, is that heretofore the mounting of the tubular glass torch to the adjustable mounts, made necessary for precise positioning of the torch, can cause cracking or breaking of the glass of the torch, necessitating replacement thereof, which can cause as much as a week""s delay in production. The glass tube of the torch is, generally, clamped to the adjustment stages using a V-block type clamp. By nature, the glass torch is extremely delicate as well as expensive, and in use, this type of clamping technique, if too tight a clamp, can cause cracking or breakage of the glass and, too loose a clamp results in process instability, thus a certain amount of operator skill is required in positioning and clamping the torch.
U.S. patent application Ser. No. 10/215,837 (hereinafter Olewicz), discloses and claims a torch assembly which is designed to and does obviate many of the aforementioned drawbacks of prior art torch assemblies. The invention of that application comprises a non-fragile mounting system having a cylindrical main glass tube surrounded by a tough and rigid tube of metal. The metallic tube is threaded at each end for receiving compression nuts, each having a ferrule such as an O-ring or a tapered ring inside. Each ferrule is a slip fit over the main glass tube of the torch and has an angled surface that butts against the end of the metallic tube. When the nuts are tightened, the sloped surfaces of the ferrules force the ferrules against the main glass tube, thereby fixing it in place within the metal tube. The metal tube in turn is clamped by a mounting clamp which is, in turn, mounted to an adjustment stage for optimum positioning of the torch. A support rail has adjustably mounted thereon one or more strain relieving clamps for holding gas or vapor delivery tubes in place to insure proper delivery of gases and vapors to the torch. More specifically, the glass torch comprises an outer main tubular body within which may be a plurality of coaxial glass tubular bodies of diminishing size for creating a plurality of gas and/or vapor delivery passages. Such nested nozzles are shown, for example, in U.S. Pat. No. 4,627,866 of Kanamori et al., and in accordance with the Olewicz invention each of the concentric delivery passages is attached to Teflon fittings connected to glass nipples formed on the nested nozzles. The stress induced in the glass main nozzle of the torch is evenly distributed over an area around the entire torch periphery and is well below critical temperature levels for glass. It has been found that hand tightening the compression nuts is more than adequate to secure the torch firmly within the glass clamp tube. Thus, the danger of too much stress being applied to the torch main tube even during temperature fluctuations, which heretofore could produce cracking or breaking, is no longer a consideration. The torch clamp tube, and not the thin glass tube, is used to mount the torch to the adjustment stages. The mounting arrangement of the invention, therefore, is more robust and safe, and provides several other benefits in addition to those just discussed. For example, the ferrule is made of elastic and resilient material such as Viton or Teflon, or other high temperature plastic and acts as a temperature compensating member during expansion and contraction of the glass and the metal. It also acts as a heat isolator allowing the gases within the torch to stay warm; acts as a vibration and shock isolator/damper, thereby protecting the glass torch; and it provides easy rotary and linear (in/out) coarse torch alignment adjustment.
The aforementioned Olewicz torch assembly overcomes many of the problems discussed in the foregoing, but it has been found that the Teflon fittings, under extremes of heat, tend to expand more than the glass of the torch, giving rise to leaks and, further, when the torch is moved, there can be relative movement between the glass ports and the fittings, giving rise to further leakage. For the most part, these leaks are small, at least to begin with, and thus can remain undetected while, at the same time, reducing the quality of the glass rods produced by the system. Thus, in order to insure quality production by the torch assembly, it is desirable that the heat effects on the fittings and movement thereof be reduced to a minimum, and early detection or leaks be achieved by the torch system.
The present invention comprises a torch housing assembly that isolates the torch from stresses originating outside the housing, e.g., mechanical stresses associated with assembly and disassembly, and stresses resulting from movement of the gas feed lines relative to the torch, e.g., movement of the torch to desired heating areas of the rod. The housing further makes possible fixed, relatively stress free, connections to the gas feed lines and allows a measure of observation of the torch and gas supply while in use, thereby facilitating detection of cracks and leaks in the torch assembly. The housing itself is substantially air tight when the torch is mounted therein
In greater detail a torch such as that shown in Olewicz is mounted in the housing, with all of the gas feeds to the individual glass tubes being contained in the housing. Each of the several glass nipples connected to the individual torch tubular members is connected by a temperature compensating Teflon fitting to one end of a gas delivery tube, which may be, for example, corrugated or accordion pleated Teflon tubing. The fitting itself is similar to those used in the Olewicz patent and comprises a stainless steel tubular threaded member having Teflon ferrules or gaskets therein for clamping the glass nipple to the Teflon gas feed tube. The other end of the gas feed tube is connected, within the housing, to a similar fitting rigidly mounted in a wall of the housing, as by welding, and the external gas delivery tube for the particular nipple is mounted on the external end of the wall mounted fitting. With such an arrangement, the gas delivery tube connected to the nipple is isolated from any exterior stresses, and there is no relative movement of the nipple and gas tube that can occur within the housing. The housing is filled with an inert pressurized gas which, if a leak within the housing occurs, prevents the chemicals and gases from leaking out of the torch. Instead, the inert gas will tend to leak into the torch, which does not alter the proper application of chemicals to the rod. An external gas mixture sensor with an internal probe is connected to a wall of the housing, and the gas within the housing can be monitored for the presence of any of the gases, e.g., hydrogen, oxygen, and HCL, used in the torch. Presence of the component gas or gases will indicate a leak sufficiently large to overcome the presence of the insert gas. Thus, the pressurized gas can be seen to perform two functions as noted.
It is desirable that the interior of the housing be heated, primarily to prevent gas condensation. To this end, the housing has a heating member, such as heating rod, projecting into the interior thereof A temperature sensor is also placed or mounted inside the housing in a location remote from the heating element. This sensor can be in the form of a thermocouple which controls the heating elements. All of these accessory elements are coupled into the interior of the housing through leak-proof coupling members which are basically the same as those for the gas delivery tubes.
The top of the housing is a transparent cover of suitable material such as a transparent, heat resistant, plastic. This enables the operator to observe the interior of the housing to monitor for leaks, cracks, or gas condensation. Even though some leaks or cracks may be small, there will, over a short time, be an observable soot build up at the location of the crack or leak, which tells the operator that, for example, one of the tubes of the torch, or the torch itself, needs immediate replacement.
These and other features and principles of the present invention will be readily apparent from the following detailed description, read in conjunction with the accompanying drawings.